This invention is an answer to a long recognized set of problems in disk drive construction which heretofore limited track density. A short explanation of disk drive construction is in order.
In the high density magnetic storage device art, the primary means for storing data is on surfaces of circular platters or disks which have been coated with magnetic media. The disks are usually mounted, centered and spaced apart, on a spindle. The spindle is most often mounted in a housing by means of a single bearing. Due to this structure, the spindle will not remain without wobble, and the amount of the wobble will vary with temperature, age, and numerous other known or presumed variables. The further from the bearing the measure is taken, the greater the wobble will be.
The magnetic transducers which read and write data to and from the disk are mounted on a comb-shaped structure the "teeth" of which would hold the transducers at appropriate locations between the platters. These teeth are called head arms and the comb is called a head arm assembly. The transducers are in or on the heads, at the ends of the head arms.
Since the transducers are thus all in a plane, no matter where they are radially over their respective disk, dedicating one transducer to reading servo tracks (the reading of which indicates its relative or absolute radial location) provides a very accurate indication of where all the transducers are, radially. Unfortunately, the disk furthest from the bearing will have a great deal more wobble in it than the one nearest the bearing, thus if you place the servo tracks and transducer on the platter nearest the bearing you may space the tracks apart very narrowly, but lose that resolution, progressively, and unpredictably to some extent, as you move to platters successively further from the bearing. So, the servo tracks would be the best predictors of transducer-to-track correspondence (across all the disks on a spindle) would be at the central disk.
While the resolution will not be as fine as if the servo transducer were on the bearing end of the spindle, it is much better than having the servo transducer at the far end of the spindle. If the servo transducer were on the bearing end, the number of servo tracks would be limited by the greatest possible wobble at the far end.
Clearly, the best location for a servo platter is where the relative displacement between the servo head following the servo track and the furthest head from the servo head track is smallest, i.e., the center platter. Thus, the relative displacement between the servo track being followed at either end is equal.
In the preferred embodiment, however, the reason for locating the head off the end platters is even more compelling. The preferred embodiment has a bearing at both ends of the spindle. Therefore the greatest wobble will be found on either end, and in all likelihood the wobble will be in opposite directions on opposite ends of the spindle.
Cross-talk problems are encountered where a central disk is used for servo, however, and because no satisfactory way within this system had been found to ameliorate cross-talk, disk drives had heretofore been built with the servo platter on one end, usually the bearing end. This invention alleviates the cross-talk problem on a central platter and allows for the use of a single-plane transducer assembly, in one case with the servo transducer on the same head arm as a read/write head and in another on a dedicated head arm. In the drive assembly in this invention, two bearings are used, one at each end of the spindle.
Cross-talk problems can occur for several reasons, including cross-talk between the wires which read the transducer pick-ups when these wires are adjacent one another, or from other transducers writing during the time the servo head is reading.
Other ways to eliminate cross talk problems may include putting the servo head on an arm apart form the head arm assembly, or, as is commonly done, putting the servo head on the last disk on the bearing end of the spindle, and dedicating that arm to the servo head. (Usually each arm carries two heads.)
This patent is not the first to describe use of a coil which is somehow inactive to cancel some cross talk problems. Perhaps the closest in general concept is the patent issued to Pizzuto, U.S. Pat. No. 4,008,493 which uses two coils of opposite polarity to cancel noise. The Pizzuto design, however is used to cancel cross talk derived from the same media, whereas the instant case solves the problem caused by writing of heads on other platters which produce a radiated field, the effect of which is to cause cross talk problems. In Pizzuto, the two coils are in the same head, whereas in the instant case the transducers are on opposite ends of the flyer, and thus relatively far from one another. In FIGS. 4, 8, and 9 of Pizzuto, the inactive transducer is shown to ride away from the media, in the instant invention, it must remain over the media as does the active transducer. Lastly, this invention uses thin film manufacturing techniques which are not similar to the techniques used to build the kinds of transducers shown in Pizzuto.
U.S. Pat. No. 4,291,352 issued to Gooch is concerned with reducing surface leakage flux induced cross talk between channels in a multichannel read head, and also does not address the radiated field cross talk problem. It also is concerned with tape drive transducers.
Of interest also is the Ohi, et al., reference, U.S. Pat. No. 3,959,824, which uses a sheild of metal between a writing transducer and the read transducer, along with a pair of oppositely wound coils to cancel the write-generated noise.
Additional designs for reducing tape transducer cross talk are shown in U.S. Pat. Nos. 3,585,314, 3,287,713 and 4,369,476, but these are considered cumulative to those previously described.
Toshimitsu, U.S. Pat. No. 4,245,268 shows the use of specially shaped coil and core structures to cancel radiated field noise. By virtue of the technology, wound core, for tape drive, it does not require that the compensating coil be in the same planar orientation as is the case with the thin film technology used with the instant invention in disk drives. The location of the coil in Toshimitsu appears to be critical, which is not the case with the instant case.
Lazzari, U.S. Pat. No. 4,184,631 applies to the magneto-resistive sensor art, but it includes a compensating magneto-resistive element. It is useful in the check reading art.